1. Field of the Invention
The present invention relates to dense wavelength division multiplexed (DWDM) optical systems using multiwavelength mode-locked (MWML) lasers as light sources and, in a presently preferred embodiment, to architectures of DWDM optical communications systems using the outputs of MWML laser sources for the transport of data and telephony services.
2. Description of the Prior Art
There has been much research conducted over the years concerning the use of dense wavelength division multiplexing (DWDM) systems to increase the bandwidth of existing and emerging fiber optic transmission systems. As shown in FIG. 1, a conventional approach has been to use a plurality of stabilized laser diodes 10 operating at or near 1550 nm with wavelength separations of 0.4, 0.8, or 1.6 nm (frequency separations of 50, 100, or 200 GHz) where each laser diode 10 is modulated at rates between 155 Mb/s and 2.5 Gb/s by wideband external modulators 12. These modulated optical carriers, or optical data streams, are combined together using an optical combiner 14 to construct a higher bit-rate optical data stream that can be amplified by one or more wideband power amplifiers 16 and can be inserted into one single-mode optical fiber 18.
Conventional DWDM optical transmission systems of the type illustrated in FIG. 1 can transmit data of very high bit rates over conventional fiber optic lines. As is apparent from FIG. 1, such conventional DWDM optical transmission systems require a plurality of light sources 10 and data modulators 12. These components are expensive and complex, and the resulting multicomponent systems are exceedingly complex and expensive. A less expensive alternative is desired.
Recently, multiwavelength mode-locked (MWML) laser light sources have been developed which generate a plurality of pulses of different wavelengths in which each wavelength emits picosecond pulses at high rates. When a conventional pulse interleaving configuration including delay line units is included within such MWML laser sources, the output optical pulse train may be multiplexed to provide very high pulse rates at each wavelength, such as those which are suitable for DWDM transmission. A MWML laser source of this type has been described by Shi et al. at a presentation entitled "Four-Wavelength, 10-GHz Picosecond Pulse Generation From an Active Mode-Locked Single-Stripe Diode Laser" given May 20, 1997, at the 1997 Conference on Lasers and Electro-Optics, and described in an associated paper entitled "20.times.5 Gbit/s Optical WDM Transmitter Using a Single-Stripe Multiwavelength Modelocked Semiconductor Laser." Commercial embodiments of such a MWML laser source are described in commonly assigned U.S. patent application Ser. No. 09/191,218, filed Nov. 12, 1998 (Attorney Docket No. SAR 13170). As described therein, a single mode-locked laser, preferably an actively mode-locked semiconductor external cavity laser (AMSECL), emits a multiplicity of fundamental optical frequency components. An RF drive signal is provided to a semiconductor optical amplifier (SOA) situated within an optical resonator such as a Fabry-Perot cavity or ring resonator by a clock source so that the SOA amplifies mode-locked pulses periodically. The SOA preferably comprises an angled-stripe InGaAsP or GaAs/AlGaAs semiconductor optical traveling wave amplifier with facet reflectivities of substantially 10.sup.-6 or less. Such low reflectivity is necessary to keep the gain spectrum of the SOA free of undulations due to SOA Fabry-Perot modes which otherwise would interfere with the generation of multiple wavelengths, since some wavelengths would be emphasized while others would be muted by such undulations. Each fundamental optical frequency component is associated with its own unique set of additional frequency components such that each fundamental optical frequency and additional frequency components make up a unique wavelength band ("comb") of frequency components. As a result, a multiplicity of combs of optical frequency components is provided by the MWML laser source. In the time domain, the MWML laser source emits pulses of overall duration approximately equal to the inverse of the spectral width of each comb in the frequency domain.
Owing to the close wavelength spacing of the optical frequencies within a given comb, each comb may be considered as a single unique wavelength source for the purposes of DWDM systems.
In the aforementioned commonly assigned U.S. Patent Application, different embodiments of the MWML laser source are described, where each MWML laser source emits multiple discrete groups of wavelengths simultaneously in a short time interval, and each group is located at a wavelength suitable to DWDM optical transmission. Feedback or feedthrough optics modules are combined with gain modules to provide embodiments suitable for DWDM transmission. The optics are coupled to one or more ports of the amplifier in Fabry-Perot or optical ring resonator configurations to provide simultaneous feedback at the plurality of wavelengths and to provide substantially identical round-trip travel times and net gains within the lasing cavity for pulses at each output wavelength.
It is desired in accordance with the present invention to develop a DWDM optical transmission system using such MWML laser sources, so as to provide a DWDM optical transmission system which has far fewer components than conventional DWDM optical transmission systems and which has the potential for much larger bandwidths. The present invention has been developed for this purpose.